Time based digital FM demodulator

ABSTRACT

A time-based, digital FM demodulator circuit receives a stream of digital samples corresponding to an analog FM waveform. The samples are provided to a zero crossing detector, which allows a counter to determine a number of clock cycles between zero crossings. The resolution of this coarse period determination is further refined by an intercept calculation, which further localizes the zero crossing of the FM waveform based on interpolation between samples on either side of the zero crossing. Accuracy of the period determination may be further enhanced by use of a sinusoidal correction filter, which minimizes error caused by the linear interpolation performed on the sinusoidal waveform. Although the FM demodulator circuit is particularly suitable for demodulation of the chroma component of a SECAM video signal, it may advantageously be applied in a wide variety of FM demodulation applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The invention generally relates to video decoders and more specificallyto composite video decoders and FM demodulators for use in decoding thecolor information of a SECAM (Sequential Couleur Avec Mémoire) compositevideo signal.

2. Description of Related Art

There is a large surge in the use of digital video devices today.Examples include: digital televisions, LCD (Liquid Crystal Display) TVs(televisions) and monitors, DVD (digital versatile disk) recorders,personal video recorders, PC (personal computer) video cards, videocapture and streaming applications, and video conferencing. In manycases, these units need to receive an analog video signal, which may beone of the composite signals, such as NTSC (National TelevisionStandards Committee), PAL (Phase Alternating Line) or SECAM; S-video;component video or RGB (Red-Green-Blue). It is then desirable to producethe proper digital output, such as eight or ten bit ITU-R (InternationalTelecommunications Union-Radio Communication) BT (Broadcast Television)656. It is preferred that all the video decoding be done in a singlechip for all of these formats. The decoder not only has to handlecomposite signals, which means it must be able to determine the chromaand luma values, but it also must handle vertical blanking interval(VBI) data and handle VCRs (video cassette recorders), which frequentlyhave unstable timing signals.

Although a number of such systems have been developed, it is alwaysdesirable to improve the output and capabilities of the particular videodecoder. For example, it is desirable to support as many video formatsas possible while maintaining a certain level of circuit simplicity,both for cost and reliability purposes. Unlike NTSC and PAL compositevideo, SECAM video includes color information that is frequencymodulated. Thus to be able to decode SECAM video, a video decoder mustinclude an FM demodulator. Historically, phase locked loops (PLLs) havebeen used for FM demodulation. However, these components are susceptibleto various forms of electrical noise, and further add to the cost andcomplexity of the video decoder circuits.

Therefore a need exists in the art for a means of performing time-basedFM demodulation to eliminate the need for the PLL. It would be furtheradvantageous to perform the demodulation in the digital domain, so as toreduce the susceptibility of the circuitry to various forms of noise anderror.

SUMMARY OF THE INVENTION

The present invention is directed to a time-based, digital FMdemodulator circuit. The FM demodulator receives a stream of digitalsamples corresponding to an analog FM waveform. These samples areprovided to a zero crossing detector, which allows a counter todetermine a number of clock cycles between zero crossings. Theresolution of this coarse period determination is further refined by anintercept calculation, which further localizes the zero crossing of theFM waveform based on interpolation between samples on either side of thezero crossing. Accuracy of the period determination may be furtherenhanced by use of a sinusoidal correction filter, which minimizes errorcaused by the linear interpolation performed on the sinusoidal waveform.Although the FM demodulator circuit is particularly suitable fordemodulation of the chroma component of a SECAM video signal, it mayadvantageously be applied in a wide variety of FM demodulationapplications.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 displays a block diagram of an exemplary personal video recorderusing an analog video decoder according to the present invention.

FIG. 2 is a block diagram of an analog video decoder according to thepresent invention.

FIG. 3 is a block diagram depicting a composite decoder portion of ananalog video decoder according to the present invention.

FIG. 4 illustrates a prior art analog time-based FM demodulator.

FIG. 5 illustrates a digital time-based FM demodulator according to thepresent invention.

FIG. 6 illustrates a frequency modulated waveform and a digitallysampled version of the FM waveform to better understand certainteachings of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, an exemplary personal video recorder (PVR) 100is shown. PVR 100 is an exemplary use of analog video decoder 102, andit is understood that the analog video decoder can be used in multipleapplications including digital televisions, LCD TVs, DVD recorders,video capture situations, and the like. A radio frequency (RF) orbroadcast signal is provided to a tuner 104. The tuner 104 provides bothvideo and audio outputs. The video output from the tuner 104 or a videosignal from an external connection is provided to analog video decoder102. The audio signal from the tuner 104 or an external audio signal isprovided to an audio decoder 106. The output in the analog video decoder102 is preferably an ITU-R BT 656 format digital signal, which may beeither an eight or ten bit signal. This digital signal from analog videodecoder 102 is provided to an MPEG (Motion Picture Experts Group) codec(coder-decoder) 108 to perform video compression in the digital domain.Similarly, the audio decoder 102 provides a PCM (Pulse Code Modulated)signal to the MPEG codec 108 to allow MPEG codec 108 to performcompression of the digital audio signal. The MPEG codec 108 in outputmode provides an ITU-R BT 656 digital stream to an analog video encoder110, which in turn produces an analog video signal output. Similarly,the MPEG codec 108 provides a PCM digital signal stream to an audioencoder 112, which provides an analog audio signal output.

The MPEG codec 108 is connected to a host bus 114 of a host CPU (CentralProcessing Unit) 116. The host CPU 116 performs processing operationsand controls the various devices located in the PVR 100. The host CPU116 is connected to flash memory 118 to hold its program and RAM (RandomAccess Memory) 120 for data storage. The host CPU 116 also interfaceswith a front panel 122. A hard drive interface 124 is also connected tothe host bus 114, with a hard drive 126 connected to the hard driveinterface. The various decoders 102 and 106 and encoders 110 and 112 arealso connected to the host bus 114 to allow control and setup by thehost CPU 116.

In operation, audio and video signals are provided to the analog videodecoder 102 and the audio decoder 106, which then provide their digitalstreams to the MPEG codec 108. The host CPU 116 programs the MPEG codec108 to transfer data to the hard drive interface 124, and thus to thehard drive 126, for storage. The host CPU 116 could at a later timedirect data to be transferred from the hard drive 126 to the MPEG codec108 for playback. Thus an analog video decoder 102 is an important partof such analog-to-digital video devices.

A block diagram of an exemplary analog video decoder 202 is shown inFIG. 2. The video signal is provided to an external capacitor 202, andis then provided to a clamp, buffer, automatic gain control (AGC) andsample and hold (S/H) block 204. Block 204 provides clamping of thevideo signal to ensure that the video signal does not exceed limits,impedance buffering and line driving, and automatic gain control andsample and hold. The output is then utilized by an analog-to-digitalconverter (ADC) 206 which does the actual analog-to-digital conversionof the video rate signals. The ADC 206 is preferably operated on asample clock, which is a free running sample clock and is not locked tothe source video in the preferred embodiment. In alternate embodiments,a source locked clock signal could be used. The output of the ADC 206 isprovided to an anti-aliasing/decimation filter 208 because preferably,the ADC 206 oversamples the video signal for increased accuracy. Theanti-aliasing portion is a low pass filter used to remove sampling aliaseffects. The decimation filter then reduces the effective sample ratedown to the desired rate, such as 27 MHz. The output of theanti-aliasing/decimation filter 208 is provided to a composite decoder210 in the case of a composite video signal such as NTSC, PAL or SECAM.The composite decoder 210 separates the luma and chroma signals andprovides these signals to a digital output formatter 212, which producesa 4:2:2, eight or ten bit signal according to the ITU-R BT 656 standard.

The output of the analog-to-digital converter 206 is also provided to alow pass filter 214 which removes any of the video content, leaving thesync signals. The output of the filter 214 is then provided to a syncdetector 216, having outputs that are horizontal and vertical syncsignals. The low pass filter 214 output is also connected to a clockgenerator 218, which is effectively a PLL and produces a source lockedclock used by other devices, if appropriate.

One portion of video decoder 102 is composite decoder 210. Oneadvantageous aspect of a composite decoder is the ability to demodulateSECAM signals. SECAM (Sequential Couleur Avec Mémoire) is a compositevideo standard generally used in France, some French-speaking countries,as well as Russia and some former Soviet republics. Details of SECAM andother composite video systems are generally known to those skilled inthe art, and thus aspects not critical in the context of the presentinvention are not repeated here.

SECAM is similar to the PAL standard used in most countries other thanthe United States and Japan. In SECAM video, the line, field, and frametiming is identical to the PAL standard. However, unlike PAL, SECAMchroma (color) information is frequency modulated (FM) rather than phasemodulated (PM). An FM modulated signal is simply a signal that willchange in frequency by the rate of the modulating signal.

All color video signals comprise three color components: red, green, andblue. These colors are the three primary colors of light, and any colorcan be made by some combination of these three colors. SECAM brightness(luma) signal Y is simply a weighted sum of the red, green, and bluecomponents. The luma signal alone can be displayed as a monochromeimage, i.e., black and white television. SECAM color informationcomprises two color difference signals Dr and Db. The Dr signal is ascaled difference between the red signal and the luma signal. The Dbsignal is a scaled difference between the blue signal and the lumasignal. Thus the original red, green, and blue components can be readilyderived from the luma signal Y and the color difference signals Dr andDb.

The color difference signals Dr and Db are FM modulated on a 4.43 MHzcenter carrier ±approximately 200 kHz. Thus to provide for demodulationof SECAM signals, an FM demodulator is required. Furthermore, it isadvantageous to provide the capability to perform FM demodulationwithout the addition of substantial additional circuitry and/or logic toa video decoder.

One technique for accomplishing this objective is to use the time-basedFM demodulation apparatus and technique of the present invention. In thecase of SECAM video signals, the bandwidth of the Dr and Db signals islimited to approximately 1.2 MHz. This relatively limited bandwidthmeans that adequate samples of the carrier are available to compute thezero crossing change of the carrier (and hence FM demodulate). Thisadvantageously allows FM demodulation to be performed using the timebase correction module that is already present in a video decoder.Although described in terms of demodulating SECAM video signals, thepresent invention is not so limited, and may advantageously be used todemodulate any FM signal.

The SECAM composite video signal is composed of luma information “Y”(brightness) and chroma information “C” (color). With reference to FIG.3, a sampled (digitized) version of the SECAM composite video signal 301is input into composite decoder 210. Specifically, the signal is inputinto YC separator 302, which subtracts the luma information from thecomposite signal. This luma signal Y is one output of composite decoder210. The color signal C, which remains after the luma signal isseparated, is passed into chroma demodulator 303. Chroma demodulator 303will separate the chroma signal C into its two components Dr and Db.Alternating lines of the SECAM signal will include either the Dr or Dbsignal. Thus a first line will be encoded, for example, with the Drcolor information, and a next line will be encoded with the Db colorinformation.

Chroma demodulator 303 is essentially an FM demodulator, with acommutation switch so that the demodulated color difference signal canbe output to the correct Dr or Db line depending on which colordifference signal is present for the current line. A typical prior arttime-based FM demodulator used in many DSP applications is illustratedin FIG. 4. An FM modulated signal 401 is input into a high gainamplifier 402. The output of high gain amplifier 402 saturates fromvirtually any input, creating a square wave 403 at its output, with thesquare wave having a period, i.e., time between zero crossings,identical to the FM modulated signal 401.

This square wave 403 serves as the input to counter 404. Counter 404also has as an input high frequency clock 410. Typically high frequencyclock 410 is about 128 times faster than period of the square wave. Thenumber of counts output from counter 404 are used to determine theperiod of the square wave by period calculator 405. This calculatedperiod is inverted by period inverter 406 to determine a frequency. Thedetermined frequency is input into summing module 407, which subtractsthe carrier frequency f_(s). The determined frequency minus the carrierfrequency is the demodulated signal 409.

A time-based FM demodulator according to the present invention isillustrated in FIG. 5. The digital samples of the FM signal 501 areinput into a zero crossing detector 502. Zero crossing detector 502creates a signal that will change state when the digital sampled datapasses through zero (i.e., changes sign). The zero crossing “trigger”signal is input to counter 506, which is driven by clock 507. Forpurposes of the video demodulation application described herein, theclock is driven at 27 MHz; however, the clock may be driven at anyappropriate frequency for other applications. In any case, the clocksignal and zero crossing pulses input into counter 506 allow the counterto determine the number of clock cycles between zero crossings, and thuscoarsely determine the frequency. The frequency is only coarselydetermined because the resolution is limited to the resolution of clock507. However, the present invention further provides for additionalresolution enhancement of the frequency determination.

The digital samples of FM signal 501 are also input along path 503 tointercept calculator 504. The zero crossing trigger signal generated byzero crossing detector 502 is also input into intercept calculator 504,causing it to store the sample value prior to the zero crossing and thesample value post zero crossing. This operation may be furtherunderstood with reference to FIG. 6. The analog FM modulated sine wave601 is represented as a series of digital samples, indicated by waveform602. The sampled values are input into the zero crossing detector 502.When zero crossing detector 502 detects the sign change between sample603 (a positive value) and sample 604 (a negative value), the outputsignal of the zero crossing detector 502 causes the intercept calculatorto retain the sample values 603 and 604 for further processing.

Intercept calculator 504 performs an interpolation between the samples603 and 604 to approximate the actual intercept 605 of the FM waveform.The calculated intercept indicates what fraction of a cycle of thedemodulation clock 507 should be subtracted from the count determined bycounter 506 to further refine the demodulated frequency. By calculatingthe intercept, and reducing the integer clock count by this fractionalvalue, the effective resolution to signal period is significantlyenhanced. Furthermore, this enhanced resolution is achieved without theneed to increase the sample frequency as typically done in prior arttime-based FM demodulators.

In a preferred embodiment, the calculated intercept determined byinterpolation by intercept calculator 504 may be further refined bysin(x) corrector 505. As noted above, intercept calculator 504determines the location of intercept 605 by linear interpolation betweensamples 603 and 604. However, the actual analog signal is a sine wave,not a linear function. The deviation between the linear function used inthe interpolation and the actual value of the intercept of the sine wavemay impose additional error on the order of several percent, whichintroduces noise into the signal. Therefore sin(x) corrector 505 takesadvantage of the edge rates and pre-emphasis of SECAM signaling toreduce the error of intercept calculation and reduce the noise in thesignal. In a preferred embodiment, this correction is performed by alookup table arrangement, although other techniques will be apparent tothose skilled in the art.

The total period (and thus frequency) of the FM waveform is thendetermined at 508 by taking the number of clock pulses counted bycounter 506 and subtracting the fraction of a clock pulse indicated bythe intercept calculation and optional sin(x) refinement. Because videodata is continuous, it is necessary to determine the moving period ofthe FM signal, which is performed by operational block 509. At thispoint, the FM demodulation is substantially complete. However,additional post processing based on known characteristics of the SECAMvideo signal may be applied to further reduce noise. For example,limiter 510 may be applied to limit the data ranges and reject spuriousfrequencies by checking the computed period based on expected signalprofiles of SECAM signaling. The limiter will reject unexpected values(based on SECAM signal profiles) further reducing noise and decodeerrors. Additionally, infinite impulse response (IIR) low pass filter511 may be used to de-emphasize certain parts of the signal that areemphasized as part of standard SECAM signaling. This filter 511 may alsobe used to integrate the FM signal, which further reduces noise in thesystem.

Thus by providing a mechanism for digital time-based demodulation of thechroma component of a SECAM composite video signal, the error/noiseperformance of a video decoder may be substantially enhanced whilereducing the cost and complexity of the video decoder. Whileillustrative embodiments of the invention have been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A method of demodulating a frequency modulated signal comprising:sampling the frequency modulated signal to produce a digitalrepresentation of the frequency modulated signal; detecting a pluralityof zero crossings of the digital representation; determining a number offixed time periods between the plurality of zero crossings; calculatingan intercept for each of the plurality of zero crossings, wherein theintercept is calculated by interpolation between a sample prior to thezero crossing and a sample subsequent to the zero crossing; anddetermining a frequency of the frequency modulated signal from thenumber of fixed time periods and the calculated intercept.
 2. The methodof claim 1 wherein determining the period of the signal from the numberof fixed time periods comprises reducing a sum of the number of fixedtime periods by a fraction of the fixed time period determined by theinterpolation.
 3. The method of claim 2 further comprising: correctingthe calculated intercept to account for non-linearity of the frequencymodulated signal.
 4. The method of claim 1 further comprising:correcting the calculated intercept to account for non-linearity of thefrequency modulated signal.
 5. The method of claim 1 wherein thefrequency modulated signal comprises chroma information from a SECAMcomposite video signal obtained by passing the SECAM composite videosignal through a luma-chroma separator.
 6. The method of claim 5 furthercomprising filtering the determined frequency to eliminate spuriousfrequencies inconsistent with a SECAM chroma signal.
 7. The method ofclaim 5 further comprising passing the determined frequency through ade-emphasis filter to eliminate effects of pre-emphasis on the signal.8. A digital time-based FM demodulation circuit comprising: a zerocrossing detector configured to receive samples of a frequency modulatedsignal and output a zero crossing indicating signal; a counterconfigured to receive a clock signal and the zero crossing indicatingsignal and to output a number of clock pulses between zero crossings;and an intercept calculator configured to receive the samples of thefrequency modulated signal and the zero crossing indicating signal andcalculate an intercept of the frequency modulated signal byinterpolation between a sample prior to a detected zero crossing and asample subsequent to the detected zero crossing; whereby a frequency ofthe frequency modulated signal may be determined from a timecorresponding to the number of clock pulses between zero crossingsdetermined by the counter minus a time corresponding to a fraction of aclock pulse determined by the intercept calculator.
 9. The digitaltime-based FM demodulation circuit of claim 8 further comprising: afilter configured to receive the output of the intercept calculator andcorrect an error introduced by the interpolation.
 10. The digitaltime-based FM demodulation circuit of claim 8 wherein the frequencymodulated signal comprises chroma information from a SECAM compositevideo signal obtained by passing the SECAM composite video signalthrough a luma-chroma separator.
 11. The digital time-based FMdemodulation circuit of claim 10 further comprising a filter configuredto receive the frequency and eliminate spurious frequencies inconsistentwith a SECAM chroma signal.
 12. The digital time-based FM demodulationcircuit of claim 10 further comprising a de-emphasis filter to eliminateeffects of pre-emphasis on the frequency modulated chroma signalaccording to a SECAM video standard.
 13. A composite decoder for use inan analog video decoder, the composite decoder comprising: a luma-chromaseparator configured to receive a composite video signal and to output aluma component of the composite video signal separately from a chromacomponent of the composite video signal; a chroma demodulator configuredto receive the chroma component and to demodulate the frequencymodulated color difference components of the composite video signal,wherein the chroma demodulator comprises: a zero crossing detectorconfigured to receive samples of a frequency modulated signal and outputa zero crossing indicating signal; a counter configured to receive aclock signal and the zero crossing indicating signal and to output anumber of clock pulses between zero crossings; and an interceptcalculator configured to receive the samples of the frequency modulatedsignal and the zero crossing indicating signal and calculate anintercept of the frequency modulated signal by interpolation between asample prior to a detected zero crossing and a sample subsequent to thedetected zero crossing; whereby a frequency of the frequency modulatedsignal may be determined from a time corresponding number of clockpulses determined by the counter minus a time corresponding to afraction of a clock pulse determined by the intercept calculator. 14.The composite decoder of claim 13 wherein the chroma demodulator furthercomprises a filter configured to receive the output of the interceptcalculator and correct an error introduced by the interpolationperformed on the frequency modulated signal.
 15. The composite decoderof claim 13 wherein the frequency modulated signal comprises chromainformation from a SECAM composite video signal obtained by passing theSECAM composite video signal through the luma-chroma separator.
 16. Thecomposite decoder of claim 15 further comprising a filter configured toreceive the frequency and eliminate spurious frequencies inconsistentwith a SECAM chroma signal.
 17. The composite decoder of claim 15further comprising a de-emphasis filter to eliminate effects ofpre-emphasis on the frequency modulated chroma signal according to aSECAM video standard.
 18. A video decoder comprising: an input modulecomprising circuits for performing one or more operations selected fromthe group consisting of: clamping, buffering, automatic gain control,and sampling and holding; an analog-to-digital converter having an inputcoupled to the output of the input module and providing ananalog-to-digital converter output; a composite decoder having acomposite decoder input coupled to the analog-to-digital converteroutput and providing a composite decoder output; a digital formatterhaving a digital formatter input coupled to said composite decoderoutput and providing a digital video signal output; a sync detectorcircuit having a sync detector input configured to receive the digitalvideo signal output and provide horizontal and vertical sync outputs;and a clock generator having a clock generator input configured toreceive the digital video signal output and provide a source lockedclock output; wherein the composite decoder comprises a luma-chromaseparator configured to receive a composite video signal and to output aluma component of the composite video signal separately from a chromacomponent of the composite video signal and a chroma demodulatorconfigured to receive the chroma component and to demodulate frequencymodulated color difference components of the composite video signal,wherein the chroma demodulator comprises: a zero crossing detectorconfigured to receive samples of a frequency modulated signal and outputa zero crossing indicating signal; a counter configured to receive aclock signal and the zero crossing indicating signal and to output anumber of clock pulses between zero crossings; and an interceptcalculator configured to receive the samples of the frequency modulatedsignal and the zero crossing indicating signal and calculate anintercept of the frequency modulated signal by interpolation between asample prior to a detected zero crossing and a sample subsequent to thedetected zero crossing; whereby a frequency of the frequency modulatedsignal may be determined from a time corresponding to the number ofclock pulses determined by the counter minus a time corresponding to afraction of a clock pulse determined by the intercept calculator. 19.The video decoder of claim 18 wherein the chroma demodulator furthercomprises: a filter configured to receive the output of the interceptcalculator and correct an error introduced by the interpolationperformed on the frequency modulated signal.
 20. The video decoder ofclaim 18 wherein the frequency modulated signal comprises chromainformation from the composite video signal obtained by passing thecomposite video signal through the luma-chroma separator.
 21. The videodecoder of claim 20 further comprising a filter configured to receivethe frequency and eliminate spurious frequencies inconsistent with aSECAM chroma signal.
 22. The video decoder of claim 20 furthercomprising a de-emphasis filter to eliminate effects of pre-emphasis onthe frequency modulated chroma signal according to a SECAM videostandard.